Elevator leveling controls



Feb. 17, 1959 H, BQRDEN 2,873,819

ELEVATOR LEVELING CONTROLS 3 39.11 JOSEPH H. fig w AT TOR NEYS United States Patent ELEVATQR LEVELING CONTROLS Joseph H. Borden, Toledo, Ohio, assignor to Toledo Scale Corporation, Toledo, Ohio Ohio, a corporation of This invention relates to automatic elevator systems and in particular to means for accurately controlling the speed of an elevator car during its final approach to a floor at which a stop is to be made.

A commonly accepted power system for driving an elevator car comprises a variable speed direct current motor mechanically connected to the hoisting mechanism of the car, a variable voltage constant speed direct current generator connected to supply driving current to the motor, and an alternating current motor drivingly coupled to the generator. Shunt field windings of the motor and the generator are supplied from an independent source of direct current, the shunt field of the motor being energized at a substantially constant level and the shunt field of the generator being energized in amounts required to provide an output voltage according to the required elevator speed. Series windings, connected for cumulative compounding, are provided in the generator to compensate for the change in generator voltage with load current being drawn from the generator so that the operating speed of the motor is substantially independent of load and varies substantially in proportion to the excitation of the generator shunt field. To secure accurate correspondence between the actual field excitation in the generator and the current flowing in the field winding compensation for the residual magnetism of the generator field must be provided. It has been customary to specially heat treat the magnetic material employed in the magnetic field circuit of the generator or provide a larger air gap to reduce the residual magnetism. Without such special heat treatment the residual magnetism of an ordinary generator field structure may supply more than enough excitation to produce a minimum elevator speed greater than that permissible during a leveling operation. Even if the residual magnetism were not great enough to provide all the excitation it is still sufiicient in ordinary generators at low field strengths to interfere with the control and produce excessively different leveling speeds for different releveling operations.

The principal object of this invention is to provide means for varying the exciting current to a generator field in accordance with the immediately prior operation of the system to provide compensation for the residual magnetism in the generator field structure.

Another object of the invention is to provide control means responsive to prior operation or the generator sys tem from modifying the field current to the generator field so as to provide a desired net level of excitation.

A still further object of the invention is to provide means for establishing different levels of exciting current in the generator field to compensate for residual magnetism remaining from dilferent immediately prior excitations of the field.

A still further object of the invention is to provide leveling circuits for an elevator system that supply difierent amounts of generator field current during leveling in accordance with the type of leveling operation being performed.

These and more specific objects and advantages are obtained in elevator leveling control circuits constructed according to the invention.

In accordance with the invention the field excitation circuit for a generator field of a variable voltage elevator drive system includes means effective during a leveling operation for varying the field current to the generator in accordance with the immediately prior excitation of the field to compensate for the residual magnetism of the field structure.

Preferred embodiments of the invention are illustrated in the accompanying drawings.

In the drawings:

Fig. I is a circuit diagram of an elevator motor control system including means responsive to the immediately past operation of the system for varying the field exciting current of the generator.

Fig. II is a circuit diagram of a generally similar system showing a different method of varying the field current according to the leveling operation being performed.

These specific figures and the accompanying description are intended merely to illustrate the invention and not to impose limitations on its scope.

In the usual elevator control system a direct current motor that is connected to the hoisting mechanism has its armature 10, shown in line 21, connected through leads 11 and 12 to an armature 13 of a variable voltage direct current generator. The armatures 10 and 13 together with the leads 11 and 12 constitute a loop circuit that includes a coil 14 of an overload relay arranged to shut down the system should an overload current occur, and generator series field windings 15 and 16 that are wound to be cumulative compound with respect to a shunt field 23 of the generator. The loop circuit also includes normally open contacts M, of a motor relay, in series with contacts H of a high speed relay. The motor relay contacts M are by-passed with a resistor 17 so that the circuit is never completely open and the high speed contacts H and series field 16 are by-passed with a resistor 18. During stand-by operation, the opening of the motor relay contacts M inserts the resistor 17 into the loop circuit to reduce the circulating current through this circuit. This resistor 17 is shorted out of the circuit as soon as the motor is called upon to perform any work. The resistance of the loop circuit is also increased during low speed operation by the opening of the contacts H to disconnect the high speed series field leaving its shunt resistance 18 in the loop circuit. The series field 15 is adjusted to provide the proper compounding for low speed operation. The series field 16 is adjusted by varying the number of turns included in the circuit to secure proper compounding for high speed motor operation.

Field excitation for the D. C. motor is provided by current flow from a positive lead 20 through resistors 21 and 22 and the coil of a field protection relay FF and the shunt field winding 23 of the elevator motor to a return lead 24. The resistors 21 and 22 are included in the circuit to reduce the field current during stand-by operation, as when the car is standing at a fioor. For normal excitation of the field, which is required for medium or high speed operation, the resistor 21 is shorted out by contacts MPN of a normal motor field relay MFN that is energized as long as the motor is called upon to operate. An increased field strength for leveling speed operation is provided by shorting out both of the resistors 21 and 22 by means of contacts MFL of a leveling speed motor field relay MFL. Thus the motor shunt field 23 is at least partially excited at all times that the system is in operation.

The variable voltage generator has a shunt field 26, line 17 that, for up operation of the car, is energized from the leads 20 and 24 by way of a circuit that may be traced from the lead 20 at line 11 through normally open ar est contacts Mot the motor, relay M and contacts BK of a brake relay BK, up field relay contacts UF in line 12, a lead 27, the shunt field 26, lead 28, in line 18, a second set of up field contacts U F shown in line 18, and a Series of resistors 29 to 34 inclusive the latter being connected to the return lead 24. For operation in the. opposite direction the up relay contacts UF are opened and the down. field relay contacts DF- are closed to reverse the QOHHQC. tions to the generator shunt field 26 at leads 27, and 28.

The excitation of the generator shunt field 26v is con.-. trolled primarily. by the number and resistance. of the series resistors 29 to 34 inclusive that are included in series tht s fis dw din The sequence of operationof thefield control asa car moves from one fioor to another may be, traced through th e. circuits appearing in the figure. Thus assuming that all of the safety switches appearing in lead 40 (line 26; of Fig. I) areclo sed currentmay flow.fromthe. positive. lead 20 through the safety switches, through an operating. coil BK ofga brake relay BK, through either. an up field relay operating coil UE or. a down field relay. operating. coil DE depending upon which ofcontacts UUorDD are closed, and; through. agate switch. 41 and landingdoor switch 42 conneeted to the return lead24. The. contacts UU or DD are closed when a time interval measured from. thela'st door opening operation expires and the closure of these contacts prepares the circuit for theup field relay coil UF- or the down field relay coil DF. Therelays opa. crating the contacts UU or DD and other relays, not shown, complete the circuits to close the car. and'hall doors and thus close thegateand landing switches41 and 42. As soon as the circuit is completed to-energize the brake coil B I(;,-and, the relay coilsUF :or DF-thelbrake relay BK closes its contacts inline 11 to complete. the.

' circuit to the generator field and a circuit .irom lead20 through abrake operating coil 43 which servesto release the brelte to .allowthe, motor. armature 101:0. operate the. hoisting mechanism. Sinceall of the resistors 29 to. 34 in-. clu sivej agestillin. series with. the generator shunt field thejexcitation level is quite low and only high enough-to provide a low speed of operation. After shorttime inter 1 qce1 er,at ing-relay successively close contactsA, A1, and A2 inlines14, 13, and 12 respectively to'successively short out'corresponding portions of the series ofresistors. Thus closure of accelerating relay contact A shorts outthe resistors32 to 34 inclusive, the closure of contacts..A1 shorts out the resistor 31, and finally the closure ofcontact A2 shorts out. the resistor 30 leaving only the resistor 29 in series with the shunt field.

To further supplement the resistors 29 to 34. in controlling the field current, .shunt resistors 45, 46 .and 47, line 16, are. included in parallel with the generator shunt field 26 Of-. these resistors the resistor 46 is alwaysin the circuit while the resistor 45.,isby-passed by normally closed acceleratingrelay contacts A2 except during highspeed operation... During leveling operations when iquicli change injfield'jstrength is, required, the parallel resistor ishinserted .into. the. circuit by'the opening'of leveling relay contacts 2L.in line" 15 and simultaneously the series're sistor 34is insertedby the opening of contacts 26 i n line 20 which open as soon as thecarj approacheswithin a small distance of a fioor a tfwhich a .st op is toibe made,

Withouttfifther control the changes .ifi speed bathe; motor'iii response to, closure of the accelerating relay. contacts A, A1 fand A Z which short out :theresistors. 30"v to; 34 are so abrupt asto be uncomfortablefforipassen; gei's thejelevator'." To minimiz'ethese sudden changes? inffield' current excitation, and I provide "more. .10"; less u i form a; lei-ationforl deceleration .aidampingi motor s arranged with its 'arm'ature'sii connected in pa fallel with; the generator shunt field during normaloperations. The dampingmotor hasits.shunt fieldSI, shown in line, 5,: continually energized from. the positive supplylead 20, by. way. ofresistors 52 and 53,- theresistor 52 .beingby-passed.

to strengthen the shunt field excitation over a stand-by level of excitation when the generator is in operation.

In series with the damping motor armature 50, are resistor 54, which is shorted out of the circuit by normally open acceleration relay contacts A, line 14, except during leveling and the first step of acceleration from a stop; normally closed high speed stopping relay contacts H51; and normally closed leveling relaycontacts LA. The contacts LA open this circuit to increase the speed of response of the generator during leveling operations. A shunt circuit for dynamically braking the armature 50 during slow downs is provided around the armature 50 by way ofhigh speedstopping relay contacts H81 and resistor 55. Furthermore, the high speed stopping relay contacts H81 in series with the armature are by-passed by resistor 56 to limit the effect of opening these contacts. The damping motor armature 50 carries a fly-wheel to increasethe-inertia of the armature and thus the effectiveness of the circuit. v In thisarrangement'the damping motor armature 50 electrically is the equivalent of a large condenser, thecapacityof which corresponds'to the inertia ofthe damp ingmotor armatureand fiy-wheel. During increases in excitation. ofthe generator shunt field 26 thecontacts' A, H51 (line 13) and LA in series with thedamping motor armature 50 are all closed to connect the armature closure ofthe accelerating relay contacts-A, A1, and A2, shorting out the resistances in series with the'shunt field, result in accelerating current flowing through the damping-motor armature 50 to absorbthe sudden increasesim current rather than forci-ng the generator fieldcurrent tochange rapidly.

During a stopping operation the sequence of operation of the accelerating relays A, A1, and A2 is reversed. As a stoppingsignal is received as the car arrives atacer tain distance from a floor at which a call is registered;

stopping relays, not shown, are energized and these relays,

which. inelude the high speed stopping relay HS1 deenergize the high speed accelerating relay A2 and then after a time delay the relay A1. As the car approaches within a fixed'distance from the floor the first'levelingrelayisenergized to release the accelerating relay'A. The first effect of the operation of the high speed stop:

ping relay HS1 is to place the resistor 55 in parallel;

with: the damping motor armature 50 to dynamically brake the armature and at the same time, by opening the contacts HSl (line 13) in parallel with the resistor 56 to insert that resistor in series, in the circuit and thus de-.

crease the efiectiveness of the damping motor armature 50 with respect to the excitation of the generator. shunt field 26 This provides a by-pass circuit, in effect; for;

dischargingthe generatorshunt field andregulates the rate at which the field is allowed to decrease.

When the car approaches within approximately twenty inches of the floor atwhich a stop is to be made the first fesriss.ofl slin sw s i operated. wh c n opens contacts- LA, line 13, in series with thedamping,

motor armature 50, so as to-take the damping, motor but oi the eircuit and at-thesame time closes contactsLA, line 21, in parallel withthe resistor 33in theseries chain alasto maiu aina e ain mo nt re t in hegenr. erator shunt field as required for the high speed; leveling;

..;-.-. ssu ume-t att e c r.- w sz raveling p; th

forthe relay. controlling the .contacts UU *sothat control e e leveling switches to be operated isllevelinga; switch l lliwhichcloses its contacts-.at:line .22 to'completc a circuitsforathe. up; .field, relayv coil. UF and, by means w of circuits not shown, it interrupts the holding circuit of the up field relay UP is transferred to the leveling switches.

As the car continues to approach the floor at high leveling speed it operates a second slow down leveling switch which serves to open the contacts 2L appearing in parallel with the shunt resistor 47, line 15, and also open contacts 2L, line 20, in parallel with the resistor 34, line 19, to further decrease the generator shunt field to provide a low leveling speed. As the car continues to approach the floor at the low leveling speed and reaches a condition nearly level with the iloor it releases the first leveling switches LU, assuming upward motion, so that that switch by opening its contacts in line 22 deenergizes the brake relay BK, to set the brake, and the up field relay UP, to deenergize the generator shunt field 26. Deenergization of the brake relay BK also closes its contacts at line 19 to connect the generator armature 13 in parallel with the generator shunt field 26. The polarity of this connection is such that any voltage generated by the generator armature 13 causes current to flow through the generator shunt field 26 in a direction to further reduce the residual field of the generator.

With the circuits so far described the final leveling speed is controlled by the resistors 29 to 34 connected in series and resistors 45 and 46 connected in parallel with the generator shunt field. As the car, moving at final leveling speed, arrives at the fioor the first leveling switch LU, assuming the car was moving up, opens to stop the car. If the car overtravels it closes a fourth leveling switch LD, which is the first to be operated when the car is moving down toward a floor.

Closure of the down leveling switch LD line 23 completes a circuit through the brake and down field relays to run the car down at leveling speed. The stopping or dead zone is determined by the two leveling switches LU and LD which are positioned so that a small movement of the car up or down energizes one or the other of the switches. If the car should move down it would again engage or operate the up leveling switch LU indicating that the car was too low and complete circuits to cause the car to move up at low leveling speed.

Because the generator shunt field excitation must be reversed when releveling from an overtravel and not reversed when releveling in the direction of travel and since the generator field structure exhibits residual magnetism it follows that the net excitation of the generator field and hence the generated voltage dilfers according to the type of leveling operation being performed. Without compensation there is also a significant difference in generated voltage or car leveling speed between the speed during initial approach and during releveling in the same direction after the car has once stopped. This is because the generator shunt field is not only disconnected from the power source by the opening of the field relays, but is also connected across the generator armature in a reverse direction so that a small field current flowed through the field thus further reducing the residual magnetism of the field. Thus without compensation there are three different leveling speeds depending upon the type of leveling operation being performed.

According to the invention, the leveling speeds for these different types of leveling operations are made substantially equal by providing control circuits that alter the field current according to the residual magnetism remaining in the field after the last previous operation of the generator. With this control it is necessary to distinguish between an initial leveling operation made prior to the first stop at a floor, a releveling operation in the direction of travel of the car and a releveling operation counter to the travel of the car as required to correct an overtravel. To provide the additional control to compensate for the residual magnetism in the field two relays are employed together with additional circuits as shown in the figure. The relays include a first stop relay ZLR, line 27, having an operating coil 2LR shown in line 27, that is energized from the supply lead 20 by way of brake contacts BK and acceleration relay contacts A whenever the car starts away from a floor on a normal interfloor run. This relay 2LR seals itself in through its contacts 2LR in line 27 so as to remain energized until the brake relay operates during the following stop. Thus this relay distinguishes between the initial leveling operation and releveling operations made after a stop.

The second relay included for the additional control,

is a magnetic latch relay ML shown at line 29 to remember the direction of travel of the car. This latch relay ML is latched into up or down condition as determined by contacts DL or UL, lines 28 and 29, whenever the second accelerating relay A1 operates during a normal acceleration. Thus if the car is running up at medium or high speed so as to operate the accelerating relay Al the magnetic latch relay will be energized for up indication and close its normally open contacts ML in series with leveling relay contact LU in line 8. Likewise, if the car is running down at the medium or high speed, the magnetic latch relay is energized for down indication, and closes its contacts in series with down leveling relay contacts in line 9. The first relay 2LR may also be known as the releveling relay since it is in a first condition when a first leveling operation occurs before a stop and is in a second condition for any releveling operations that may occur.

To secure the desired leveling speeds with variable voltage generators having normal to high residual magnetism in their field structures often requires that the field current be reduced nearly to zero or, in some cases, actually reversed during the first leveling operation because the residual magnetism of the field structure is at a maximum level at such time. direction of travel requires slightly more field current because immediately prior to such releveling the field circuit has been disconnected from the voltage source and a small reverse current from the generator armature has been passed through the field to reduce the generator voltage during stand-by. Therefore, a releveling operation in the same direction requires more field current from the power source.

A releveling operation in a direction counter to the previous direction of travel requires still more field current because in this case the field current must oppose the residual magnetism and overcome it to the extent that the net excitation of the field is of the proper amount and in the proper direction.

Compensating current for compensation of the residual magnetism may be obtained either from a separate current source connected in parallel with the circuit comprising the generator shunt field and the direction relay contacts UP and DF or, if the required compensating current is small enough, it may be obtained as shown in Fig. 11, by shorting out portions of the resistors 29 to 34 durin releveling operations.

may provide reverse current through the generator shunt field during the first leveling operation when the residual magnetism acting alone would provide too high a leveling speed. In this case a separate current source comprising a transformer 60 and rectifier 61 supplies direct current through a resistor comprising sections 62, 63 and 64 to the generator shunt field through the direction determining contacts UF or DP. Normally closed contacts A1 of the second acceleration relay A1 are included to open this circuit during normal high speed operation and close the circuit only during the low speed and leveling operations. The regular direct current supply source connected through leads 20 and 24 cannot be used for this compensating circuit because it is of the wrong polarity.

The added circuit comprising the transformer 60 and rectifier 61 supplies a counter or bucking current through the generator shunt field and, when the resistance of the A releveling operation in the In Fig I the parallel. arrangement is shown since it has the advantage that itasve sie verse the current through the generator shunt" field. A

relatively high. voltage transformer andrectifier are ern-' plOYfid tOsuppIy current through a relatively. large'resisltance 62f 63'and16'4 to avoid placing a low resistance shuntl in parallelwith the generator shunt field'which' would'tend toslow: up theresponse of the field current.

to changes in resistance in 'theicontrol circuits. During the initial leveling. operation'before the first stop is actuallyjmadela maximtun current is required from the auxilia'ry circuit or separatecurrent source to reduce to a minimum or reverse the-current in thc'g'enerator shunt field. Thismaximum' bucking'current is provided by closingfcontact's of 'theleveling auxiliary relay, which are; closed during any' leveling operation, and closing: contactsJLR of'the'releveling relay to' short out resistors 63"and64until the'icarrnakes'a first stopin approaching.

the,.fl o,o'r. This;circuit..condition provides a minimum current flow in .the'generatorshuntfield 'or a reverse current fi'ow as'may beLrequired'to reduce'the net excita= tionof'th'e field'to thatfcorresponding to the desired low leveling speediv After the car; makes its'first stop and releases the releveling relay 2LR at least both resistors 62 and 63farejincludedv in .theseparate currentsource circuit to reduce the bucking current 'supplied'to the generator field during any subsequent releveling at that stop; If a releveling operation inthev direction of travel is required, levelingrelay.contacts LU or ID, lines 8 or 9; and the magnetic latch relay -contacts ML for the same direction will; both be.closed'to' complete'a by-pass circuit around the, resistor '64fr'om theresistor 63 through low speedleveling contacts2L, line 9; to the generator shunt field. This circuit thus'reducesithebucking current applied to the generator .shunt field'to allow' a larger net current as mayberequired"tobring the net field'excitation up to the desired 'amount'for the same direction releveling operation':

If the 'car'oversh'oots thefioor the releveling operation is *in' .adirection" opposite to the direction oftravel. In this situation'the circuit through the'leveling switch contacts LU or LD and the magnetic latch relay contacts ML, linesj8"or"9,.cannot be completed so that all three resistor" sections 62; 63 and 64 are in circuit thus providingya minimumof'bucking current through the generator "shunt field.

With ordinary commercial'components the circuit is adjusted by making the resistor 62'of approximately 1000 ohms and'then adjustingthe resistor 34 of the seriesof resistors sothat witha balanced load the car will level correctlyat'a'floor on its 'first'approach. Next the resistor'63"'isadjusted sothatifthe car is first stopped an inchor'two" short of the floor and allowed to level it will 'approachthe'fioor'at the correctlow leveling speed. Thisadjustmentcompensates forthe loss of residual magnetism-in the'generator shunt field as a'result of openingfthe circuit and; connecting the shunt field 26 across the generatorarmature 13 when the first stop is made. Final1y,' the:resistor '64 is adjusted to provide the correct lvelingspeed for reverse=releveling when thecar moves pastthe'floor; because of overshoot or change in load, and then "returns or relevels.

These compensating circuits not only make it possible to'"use'a-commercial'grade direct current machine for the generator'but also provide a quality of performance that is'equal to or better than the best'performance obtainable with low residual field generators-commonly used in clevator'dr'ive systems.- It is easier to balance a system for a large' known amount-oi? residual magnetism of a cornmercialgrade direct' current'machine than it is to build a" magnetic field circuit having-sufficiently low residual magnetism to function satisfactorily without compensation:

The'circuit'shown in Fig. H'is' another method of ac-' compli'shing the same result ofcompensating for the resid V 8" ual magnetism in the'magnetic circuit of a generator. The circuit shown in Fig. II is the same as that in Fig. I'

exceptfor the elimination of the transformer 60'with the rectifier 61 and thecircuit associated therewith and the substitution ofadditional resistance steps in the series'of resistors'29 to 34i Because of the similarity-betweenthe diagrams'thep'arts shown in Fig. IIthat correspond to similar parts in Fig. I are givenreference numerals of lOO'plus theoriginal reference numeral. Thus the motor armature 10" of Fig. I appears as a motor armature of Fig. II.

Since the m'ere'insertion of resistance in series with a shunt field supplie'd'from' a constant voltage supply cannot provide a reversal of. exciting currentbut only reduce the exciting 'current'toward's zero, this particular circuit is ap-' plicable only. to those generators inwhich the residual magnetism of the magnetic field provides a leveling speed less than that desired; In other words, this circuit cannot reduce 'thelow'speed of the car below that speed'fixed'by' the'residual magnetism of the field.

In this'circuit a magnetic latch relay ML that remem-' bers the last direction of high speed operation of the system is shown with its'contactsML, lines 112 and'113, in positions corresponding to downward travel of the car.- In lines 112 and 113'th'e magnetic latch relay conwhile the-magnetic latch relay contacts ML that are "open" as the car'moves down are-in series withld'own'leveling" switch contacts LD L In this circuit, a" tap on: the

resistor 134 is adjustedwiththeremainderof the resistors" 129-134" in series to provide the correct field excitation for lowleveling speed as'the car closely approaches a fioor before making its'stop. This requires the minimum current flow in the-generator shunt field and is provided" by'havinga maximum amount of resistance in the circuit. Asthe car makes its first stop 'it releases the releveling relay ZLR, line 124, and it in turn closes its contacts 'ZLR at line112 to short out-that portion of the resistor 134 between a second tap lil'and the tap 170. Shorting out the portion of the resistor 134 between the taps'17 1'and" 170 increases the current through the generator shunt field in the amount required to provide the 'correct'l'ow leveling speed once the residualmagneticfield has been decreased by'the last opening of the field'circuit. This providesfor proper releveling operation when the releveling is in the. direction of travel of the elevator car.

In the eventthat the car while traveling down overshoots the level position andcloses the up levelingswitch- LU, it closes itsjcontacts LU," line-'113'to complete a'cir-'- cuitfrom anothertap 172 on the resistor 134 through the now closed contacts LU'and closed magnetic latch-relay contacts ML, line 113, and now-'closed'releveling relay" contacts 2LR' and'thus' shorts out the major-portion of the resistor-134; This increases the current flow through" the generator shunt field'to an amount'sufiicient to-over--' come the residual magnetism and provide enough volt-- age to drive themotor armature 110-at the proper Iow leveling speed; It will berecalled thatona releveling operation in the reverse direction, that is against thegeneral direction of travel of the car, the generator shunt field isreversed with respect to the-supplypotential so thatthe magnetic field of the fieldcurrent is in opposition to the residual magnetism producedby the last previous operation.

In the above circuits the releveling relay' 2LR' consti-'- tutes means for determining when the'first stop has been made during'the leveling ata-fioor and thus distinguishes between the initialfleveling' operation and relevelingoper== ations following the first stop; The particular meanswfi operation. of the releveling'relay is illustrative onlyand manyyarious modificationsin the mode of'operatiorrttraybe-made' without changing thefunction of this relay:

In like manner, the magnetic latchrelay ML, thatis' employed'toremember'thedast previous operation ofithe" system and hence the state of the generator magnetic circuit insofar as residual magnetism is concerned, may be operated from any of a number of sources of signal in the control system. The directional relay contacts UL and DL and the accelerating relay contacts A1 provide a simple direct method for operating this relay. However, any circuit that is responsive to the last substantial excitation of the generator shunt field would be equally satisfactory. By substantial excitation of the generator shunt field is meant any excitation level sufficient to establish a residual magnetic field in the direction of the impressed excitation.

The circuits as described thus provide means for accurately compensating for the residual magnetic field in a generator that drives an elevator hoist motor and makes it possible to accurately adjust the leveling speeds regardless of whether the leveling be in the direction of movement of the car prior to a stop, a releveling operation in the direction of travel, or a releveling in the direction counter to the direction of travel of the car.

Various modifications may be made in the circuits with out losing the advantages obtained by compensating for the residual magnetism of the generator shunt field during the various leveling operations of an elevator system without departing from the scope of the invention.

Having described the invention, I claim:

1. In an elevator system, in combination, an elevator car that serves a plurality of floors, a variable speed direct current motor for moving the car, a variable voltage generator electrically connected to the motor for supplying driving current thereto, a constant voltage source of direct current, a series circuit comprising a field winding of the generator and at least one resistor connected across the constant voltage source, control means for incrementally varying the resistance of the resistor in the series circuit to vary the generator voltage, and means responsive to the last previous run of the motor for additionally varying the resistance of said resistor.

2. In an elevator system, in combination, an elevator car that serves a plurality of floors, a variable speed direct current motor for moving the car, a variable voltage generator electrically connected to the motor for supplying driving current thereto, a constant voltage source of direct current, a series circuit comprising a field Winding of the generator and at least one resistor connected across said source, control means for varying the resistance of the resistor in accordance with the desired motor speed, and means responsive to the direction of excitation and the previous run of the motor connected to said series circuit for varying the current flow in the field winding.

3. In an elevator system, in combination, an elevator car that serves a plurality of floors, a variable speed direct current motor for moving the car, a variable voltage generator electrically connected to the motor for supplying driving current thereto, a constant voltage source of direct current, a series circuit comprising a field winding of the generator and at least one resistor connected across said source, control means for varying the resistance of said resistor in accordance with the desired motor speed, a second source of voltage, resistor means connecting the second source of voltage to said field winding to pass current therethrough in opposition to said constant voltage source, and means for varying the resistance of said resistor means in accordance with the last run of the motor and the direction of field excitation.

4. In an elevator system, in combination, an elevator car that serves a plurality of floors, a variable speed direct current motor for moving the car, a variable voltage generator electrically connected to the motor for supplying driving current thereto, a source of constant voltage direct current, said generator having a field winding, a circuit including at least one resistor for connecting the winding to the constant voltage source, means for varying the resistance of the resistor to vary the field current of the generator and thus the speed of the motor, said means operating to increase the resistance of the resistor as the car approaches a floor at which a stop is to be made, auxiliary means for varying the field current, control means responsive to the last previous motor operation and the direction of generator field excitation for operating the auxiliary means, said auxiliary means providing sufficient field current variation to compensate for the residual magnetism of the generator field whereby the generated voltage is proportional to the field current as fixed by said resistor and constant voltage source.

5. In an elevator system, in combination, an elevator car that serves a plurality of floors, a variable speed direct current motor for moving the car, a variable voltage generator electrically connected to the motor for supplying driving current thereto, a control field on the generator, first means for varying the excitation of the control field to provide variable speed operation of the motor including a speed suitable for leveling the car at a floor, and second means for varying the excitation of the control field during leveling operations, said second means having elements selectively responsive to normal stops at a floor, stops requiring further leveling movement in the direction of travel, and stops requiring further leveling movement in reverse to the direction of travel for varying the excitation according to the kind of stop.

6. In an elevator system, in combination, an elevator car that serves a plurality of floors, a variable speed direct current motor for moving the car, a variable voltage generator electrically connected to the motor for supplying driving current thereto, a control field on the generator, first means for varying the excitation of the control field to provide variable speed operation of the motor including a speed suitable for leveling the car at a floor, and second means for varying the excitation to compensate for residual magnetism in the generator field, said second means cooperating with said first means to provide minimum excitation for the generator as the car levels with the floor, an increased excitation for leveling if the car first stops before reaching the floor, and a further increased excitation if the car first stops beyond the floor.

7. In an elevator system, in combination, an elevator car that serves a plurality of floors, a variable speed motor for moving the car, a variable voltage generator electrically connected to the motor for supplying driving current thereto, a control field Winding on the generator, a direct current source of constant voltage to which the control field winding is connected, at least one resistor connected in series with the control field winding, control means for shorting out portions of said resistor to increase the speed of the motor from a leveling speed, means responsive to the car stopping short of a position level with a floor for shorting out a first increment of said resistor, and means responsive to the car stopping beyond a position level with a floor for shorting out a second increment of said resistor.

8. In an elevator system, in combination, an elevator car that serves a plurality of floors, a variable speed motor for moving the car, a variable voltage generator electrically connected to the motor for supplying driving current thereto, a control field winding on the generator, first direct current means including a series resistance connected to the control field winding to supply current to the field winding according to a predetermined pattern, a second direct current means including a second series resistance connected to supply current to the control field in opposition to the first direct current means, means for shorting out a portion of the second series resistance when the car relevels in the direction of initial approach to the floor at which a stop is to be made, and means for shorting out a larger portion of the second series resistance while the car is first approaching a position level with the floor.

9. In an elevator system, in combination, an elevator car that serves a plurality of floors, a variable speed motor trically connected to the moto'r'fdr su plying'driving cur-' rent thereto, a control field winding on the generator, first means for supplying field current to the control field according to a predetermined pattern, and means for'rnodifying the pattern at leveling speeds,said means comprising a first relay responsive to the direction of the last previous substantial excitation of the control field and relay means responsive to the" first sto on a roaching a,"

floor, said modifying means serving to increase the strength of the control field current during releve'ling attempts after the car has once stopped in amounts to 5 compensatefo'rthe residual magnetism in the field.

No references cited.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 2,873,819 February 17, 1959 Joseph H. Borden It is hereby certified that error appears in the above numbered patent In the drawings, Sheet 2, Fig. II, at line 109, cancel the short lead that appears immediately above the reference numeral "134" and that connects the right end of resistor 132 directly to the return lead.

Signed and sealed this 7th day of July 19596 (SEAL) Attest:

KARL H. I At AXL ROBERT C. WATSON testing fficer Commissioner of Patents UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 2,873,819 February 17, 1959 Joseph H.. Borden corrected below.

i In the drawings, Sheet 2, Fig. II, at line 109, cancel the short lead that appears immediately above the reference numeral "134" and that connects the right end of resistor 132 directly to the return lead.

Signed and sealed this 7th day of July 1959,

(SEAL) Attest:

KARL H. AX IN A'bt L ROBERT C WATSON eating offlcer I Commissioner of Patents 

